Explain the concept of regression testing.

Regression testing is a software testing technique that aims to ensure that changes or modifications made to a software application do not introduce new defects or issues into previously tested functionality. It involves retesting the existing functionalities of the software to verify that they still work as expected after any changes have been made.

The concept of regression testing is based on the assumption that any change in the software, whether it is a bug fix, enhancement, or new feature, has the potential to unintentionally introduce new bugs or impact the existing functionality. Therefore, regression testing is performed to mitigate the risk of regression, which refers to the reoccurrence of previously fixed defects or the introduction of new defects due to changes made in the software.

The main objectives of regression testing are to ensure that the software remains stable and reliable, to validate that the changes made have not adversely affected the existing functionality, and to maintain the overall quality of the software. It helps in identifying and fixing any defects that may have been introduced during the development or modification process.

Regression testing can be performed at different levels, including unit testing, integration testing, system testing, and acceptance testing. It involves executing a set of test cases that cover the critical functionalities of the software, as well as the areas that are most likely to be affected by the changes. The test cases may be selected from the existing test suite or may be newly created to specifically target the areas impacted by the changes.

There are various techniques and approaches to regression testing, such as retesting all the test cases, selecting a subset of test cases based on risk analysis, prioritizing test cases based on the impact of changes, and using automated regression testing tools. The selection of the regression testing approach depends on factors like the complexity of the changes, time constraints, available resources, and the criticality of the software.

In conclusion, regression testing is a crucial part of the software testing process that ensures the stability and reliability of the software by verifying that changes made to the software do not introduce new defects or impact the existing functionality. It helps in maintaining the overall quality of the software and mitigating the risk of regression.